Why changing camera's far clip does not seem to clip the distance for depth map rendings?
This is essentially just a matter of interpretation and intended meaning. When the depth reports 10000000000 it’s not actually intended to represent an actual distance, rather ‘infinity’ (but it’s a suitably large enough value to essentially be regarded as that) and so it represents the rays not actually hitting anything. The camera clipping distance is effectively a tool to allow the render to be optimised (so we don’t consider details that are too distant to make a noticeable difference to the final render. It does affect which surfaces are considered for rendering but the ‘infinite’ depth is still relevant to indicate ‘nothing was hit’.

When interpreting a depth map it’s much clearer to interpret a single value (“infinity”) as an indicator that nothing was hit than a ‘variable’ value based on the camera clipping distance - maxing out at the clipping distance would require additional knowledge as to what the actual clipping distance was set to for that particular render before you could determine whether a point hit a surface or not.

If you know your clipping distance you should be able to manually limit the upper bounds with something like `min(9999,max(R))’ - ie, get the maximum value from R and then take the minimum of that and some constant based on your clipping distance.